1. Field of the Invention
This invention relates to embedded microprocessor systems, and more particularly, to a program repairing apparatus for use on an embedded microprocessor system to repair any faulty program segments that are stored in a permanent data storage unit, such as a mask ROM (read-only memory) incorporated in the embedded microprocessor system.
2. Description of Related Art
Conventionally, microprocessor systems are constructed by manufacturing microprocessors, memory units, and I/O (input/output) devices as individual units in separate IC devices and then mounting these IC devices on printed circuit boards (PCB). With the advance in semiconductor technologies, however, today's microprocessor systems can be constructed all on a single semiconductor chip. Such a microprocessor system is usually referred to as an embedded microprocessor system.
FIG. 1 is a schematic block diagram of the core microprocessor of a conventional embedded microprocessor system. As shown, the embedded microprocessor system includes a ROM (read-only memory) unit 800, a program counter 81, a decoding unit 82, an execution unit 83, and a control unit 84. The ROM unit 800 is used for permanent storage of routinely executed programs by the execution unit 83. The sequential output of the program counter 81 is used as access addresses to gain access to specific storage locations in the ROM unit 800 so as to read out the corresponding program codes from the same. The program codes from the ROM unit 800 are subsequently processed by the decoding unit 82 so as to convert it into suitable form that can be executed by the execution unit 83. During execution of the program, the execution unit 83 can change the output of the program counter 81 in response to various conditions so as to fetch certain special program routines located on specific addresses of the ROM unit 800. All of the foregoing actions are under control by the control unit 84. Detailed structures and functions of this embedded microprocessor system are all conventional so description thereof will not be further detailed.
The ROM unit in an embedded microprocessor system is usually realized by a mask ROM since the cost of mask ROMs are significantly cheaper than other kinds of nonvolatile memories. The program codes are permanently written into the mask ROM through the so-called mask programming process that requires to manufacture a mask layer first, then uses the mask layer in a semiconductor manufacturing process to complete the building of the program codes in the mask ROM. Once stored, the program codes can not be arbitrarily modified or updated unless another mask programming process is performed on the mask ROM. However, the use of the mask programming process to modify or update programs stored in the mask ROM is quite a laborious and time-consuming process to carry out. Finished products of embedded microprocessor systems may be discarded because the program codes stored in the mask ROM have bugs. There exists, therefore, a need for a program repairing apparatus which can repair any faulty program segments in the ROM unit of an embedded microprocessor system without the use of mask programming process.